Replace Monaco with CodeMirror (#36764)

- Replace monaco-editor with CodeMirror 6
- Add `--color-syntax-*` CSS variables for all syntax token types,
shared by CodeMirror, Chroma and EasyMDE
- Consolidate chroma CSS into a single theme-independent file
(`modules/chroma.css`)
- Syntax colors in the code editor now match the code view and
light/dark themes
- Code editor is now 12px instead of 14px font size to match code view
and GitHub
- Use a global style for kbd elements
- When editing existing files, focus will be on codemirror instead of
filename input.
- Keyboard shortcuts are roughtly the same as VSCode
- Add a "Find" button, useful for mobile
- Add context menu similar to Monaco
- Add a command palette (Ctrl/Cmd+Shift+P or F1) or via button
- Add clickable URLs via Ctrl/Cmd+click
- Add e2e test for the code editor
- Remove `window.codeEditors` global
- The main missing Monaco features are hover types and semantic rename
but these were not fully working because monaco operated only on single
files and only for JS/TS/HTML/CSS/JSON.

| | Monaco (main) | CodeMirror (cm) | Delta |
|---|---|---|---|
| **Build time** | 7.8s | 5.3s | **-32%** |
| **JS output** | 25 MB | 14 MB | **-44%** |
| **CSS output** | 1.2 MB | 1012 KB | **-17%** |
| **Total (no maps)** | 23.3 MB | 12.1 MB | **-48%** |

Fixes: #36311
Fixes: #14776
Fixes: #12171

<img width="1333" height="555" alt="image"
src="https://github.com/user-attachments/assets/f0fe3a28-1ed9-4f22-bf25-2b161501d7ce"
/>

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/js/modules/codeeditor/linter.ts

@@ -0,0 +1,39 @@
+import type {CodemirrorModules} from './main.ts';
+import type {Extension} from '@codemirror/state';
+
+/** Creates a linter for JSON files using `jsonParseLinter` from `@codemirror/lang-json`. */
+export async function createJsonLinter(cm: CodemirrorModules): Promise<Extension> {
+  const {jsonParseLinter} = await import(/* webpackChunkName: "codemirror" */ '@codemirror/lang-json');
+  const baseLinter = jsonParseLinter();
+  return cm.lint.linter((view) => {
+    return baseLinter(view).map((d) => {
+      if (d.from === d.to) {
+        const line = view.state.doc.lineAt(d.from);
+        // expand to end of line content, or at least 1 char
+        d.to = Math.min(Math.max(d.from + 1, line.to), view.state.doc.length);
+      }
+      return d;
+    });
+  });
+}
+
+/** Creates a generic linter that detects Lezer parse-tree error nodes. */
+export function createSyntaxErrorLinter(cm: CodemirrorModules): Extension {
+  return cm.lint.linter((view) => {
+    const diagnostics: {from: number, to: number, severity: 'error', message: string}[] = [];
+    const tree = cm.language.syntaxTree(view.state);
+    tree.iterate({
+      enter(node) {
+        if (node.type.isError) {
+          diagnostics.push({
+            from: node.from,
+            to: node.to === node.from ? Math.min(node.from + 1, view.state.doc.length) : node.to,
+            severity: 'error',
+            message: 'Syntax error',
+          });
+        }
+      },
+    });
+    return diagnostics;
+  });
+}